I'm currently trying to get Zimbra in the door of a client that is currently on Exchange and is having a TON of spam/av issues. They are hesitant to move to a new product but I was hoping there was a way to put Zimbra in front of the Exchange server to scrub mails and in the case of a failure on the Exchange server at least queue the mail until the Exchange server is back up. Is this possible?

Admittedly this type of email routing is not my forte although I've tried to search the forums I can't quite find an answer.